1. Field of the Invention
The present invention generally relates to a handover method in a terminal supporting a multimode, and more particularly to a method for minimizing current consumption when a handover is performed between networks which provide communication services using different mobile communication technologies, and to a terminal for use in such a method.
2. Description of the Related Art
At present time, a mobile communication scheme is evolving from a current 2nd generation mobile communication system, such as a Code Division Multiple Access (CDMA) system and a Global System for Mobile Communications (GSM), to a 3rd generation network, such as a Wideband Code Division Multiple Access (WCDMA) system. Currently, since the 2nd generation mobile communication system is employed in widely distributed service areas, and is completely furnished with infrastructures, the 3rd generation network is expanding its territory while transitionally using the infrastructures of the 2nd generation network. Therefore, in such a transitional situation, a terminal provided with services specific to the 3rd generation network, is being manufactured as a multimode terminal supporting the existing 2nd generation system.
An important feature of the multimode terminal is to support an idle handover function, which enables switchover between a WCDMA mode and a CDMA mode in an idle state, and a traffic handover function of switching the terminal's mode to the CDMA mode without interruption when the terminal enters a WCDM shadow area in the middle of a call. These functions are economical in that they take advantage of a CDMA network belonging to the 2nd generation system. However it is difficult to internally implement hardware and software in a terminal because the handover should be performed between different modems in a short time.
Operations in a terminal performing an on-call handover between a WCDMA mode and a CDMA mode, will be described with reference to FIG. 1. In order to perform the handover to the CDMA mode in the middle of a WCDMA call, control signals necessary for the handover are transferred in a state where both of the WCDMA and CDMA modems in the terminal operate.
Referring to FIG. 1, if a terminal 100 operates in a WCDMA mode through a WCDMA base station 160 in an overlapping area of a WCDMA network, and enters an area belonging to only a CDMA network, the terminal 100 searches for a CDMA base station because the WCDMA base station 160 is not detected, and then switches its mode to a CDMA mode to complete roaming when the CDMA base station 150 is detected. To the contrary, if the terminal 100 is operating in the CDMA mode when entering the overlapping area, the terminal 100 performs a handover according to predetermined conditions in order to switch its mode to the WCDMA mode again.
Such a dual-mode terminal supporting CDMA and WCDMA modes and using a single antenna is shown in FIG. 2 When the terminal is located in a WCDMA network, a CDMA RF unit 220 and a CDMA modem 240 are temporarily powered off whereas an antenna, a diplexer 210, a WCDMA RF unit 230 and a WCDMA modem 250 are powered on, and thus the terminal operates in the WCDMA mode. In contrast with this, when the terminal is located in a CDMA network, the constituent parts are inversely powered on or off, and thus the terminal operates in the WCDMA mode.
A controller unit 200 corresponds one-to-one to each modem 240, 250 through a switch 260, and one modem is powered off while the other modem operates so as to prevent power consumption. For intercommunication, the modems 240, 250 are connected to each other via an interface 270.
As stated above, the terminal supporting WCDMA and CDMA modes uses two modem chips which supports different mobile communication services according to corresponding modes, respectively. However, if a handover occurs in the middle of a call, the terminal internally powers on the two modems, which result in current consumption one and half times to twice more than that in an idle state.
This process will be described below with reference to FIG. 3. If a call starts in the WCDMA mode, the controller unit 200 transfers a CALL ORIGINATION message to the WCDMA modem 250 in step 300. The WCDMA modem 250 then transfers a CALL SETUP message to the WCDMA base station 160 in step 305. In response to this, the WCDMA base station 160 transfers a CALL CONNECT message to the WCDMA modem 250 in step 310, and the CALL CONNECT message is transferred to the controller unit 200 via the WCDMA modem 250 in step 315.
In step 320, the controller unit 200 establishes a channel for a voice call to perform the call and, in step 325, transfers a CDMA POWER ON message for powering on the CDMA modem 240 to the CDMA modem 240. At this time, since the controller unit 200 communicates with the WCDMA modem 250, in step 330, the CDMA modem 240 does not communicate directly with the controller unit 200, but communicates with the WCDMA modem 250 to transfer a CDMA POWER ON INFORM message informing that the CDMA modem 240 is powered on. Subsequently, in step 335, the CDMA modem 240 receives a GO LOW POWER MODE message from the WCDMA modem 250 and, in step 340, enters a low power mode and waits in an idle state.
In the low power mode, modem parts for processing system acquirement procedures and related protocols do not operate. Nevertheless, if the CDMA modem 240 operates in the low power mode, the terminal consumes an additional current of about 10 to 20 mA. If a handover does not occur and the WCDMA call is completed, the controller unit 200 powers off the CDMA modem 240 to prevent power consumption. If a WCDMA call reopens, the controller unit 200 powers on the CDMA modem 240 and switches the CDMA modem 240 to the low power mode. In this way, the CDMA modem 240 is repeatedly powered on and off.
The reason why the CDMA modem is powered on and is switched to the low power mode for every WCDMA call is that, otherwise a time for switching a one-to-one communication path between the controller unit and the modem from WCDMA to CDMA, and a time during which the controller unit intermediately controls the switching additionally occur. Also, if a time till which the CDMA modem is powered on and initialized, is added to the additional time, the overall handover processing time is lengthened, which may result in failure in the handover. Moreover, since the WCDMA system cannot detect an exact point of time when the terminal performs a handover, it cannot transfer a message, which instructs the terminal to get ready for the handover, at a certain time before the handover occurs.
As stated above, if a WCDMA call starts, a terminal of the prior art inevitably powers on a CDMA modem and operates it in a low power mode in order to get ready for a handover to CDMA. However, if the CDMA modem is powered on whenever a WCDMA call starts, a current used at the time of the WCDMA call increases, and finally the overall power consumption of the terminal increases, which causes a service time of the terminal to be shortened.